In recent years, as a method for polishing a surface of a silicon wafer, CMP (chemical mechanical polishing) which is carried out so as to relatively rotate the silicon wafer and an abrasive cloth while supplying a polishing liquid in which free abrasive grains such as silica grains are contained in an alkaline aqueous solution, is common. CMP has been known by that a mechanical polishing action by free abrasive grains and a chemical polishing action by an alkaline aqueous solution are combined, to be able to obtain high flatness with respect to the surface of a silicon wafer. In this CMP processing for silicon wafer, usually, polishing is carried out through multiple steps from rough polishing to final polishing.
The rough polishing at an initial step is carried out with the purpose of polishing a silicon wafer to be a desired thickness. The polishing is carried out under the condition of a relatively high polishing rate by use of a hard abrasive cloth of polyurethane or the like, that is, the polishing is carried out so as to reduce the variation in thickness of the silicon wafer after the polishing, to achieve flattening. In this rough polishing process, the polishing processing may be carried out so as to change the type of an abrasive cloth and the size of free abrasive grains, while splitting the amount of polishing the silicon wafer (elimination stock amount) into multiple steps (for example, 1 to 3 steps) in some cases.
The final polishing in the final step is carried out with the purpose of improving the roughness on the silicon wafer surface, and the polishing is carried out so as to reduce the variation in nano-surface roughness on the silicon wafer surface such as nanotopography or haze by use of a soft abrasive cloth such as suede, and micro-sized free abrasive grains. In the same way as the rough polishing process, in this final polishing as well, the polishing processing may be split into multiple steps to be carried out while changing the type of abrasive cloth and the size of free abrasive grains, in some cases.
However, in the case where rough polishing is carried out by use of a polishing liquid containing free abrasive grains (a slurry), a high polishing rate and a certain level of flatness of a wafer may be obtained. Meanwhile, new process damage has been brought about to the surface of the silicon wafer by a mechanical action during the polishing. Further, due to aggregation of the free abrasive grains in the polishing liquid, defects caused by a process such as micro-scratching have occurred on a polished surface of a silicon wafer. In many cases, process damage occurring in the rough polishing process and defects caused by a process are not eliminated even in the subsequent final polishing process, and in order to completely eliminate those, it is necessary to considerably increase the amount of polishing in the final polishing.
Then, as a conventional technology for solving this, for example, the invention disclosed in Patent Document 1 has been known. This invention is to polish the surface of a silicon wafer by use of a polishing liquid composed of an alkaline aqueous solution of pH8.5 to 13, which contains a water-soluble silicate component and an alkaline component, and substantively does not contain free abrasive grains. In this way, because free abrasive grains are not used, process damage to the surface of the silicon wafer is reduced. In addition, because the silicic acid molecules are coupled with silicon atoms on the wafer surface coupled with a hydroxyl group to form condensed silicate and accelerate elution of the silicon atoms into the alkaline aqueous solution, its polishing rate is therefore increased.    Patent Document 1: Japanese Published Unexamined Patent Application No. H9-306881.